This invention relates to data storage devices which record and/or retrieve data recorded on rotating optical data recording discs. The data is stored on the discs in spiral or concentric tracks and is recorded or retrieved by modulation of a beam of light focused along the tracks on the disc. In particular, this invention provides an improved means of focusing and positioning a focused spot on the desired track.
Optical data storage devices usually include an optical system comprising lasers, collimating lenses, beam shaping prisms, beam splitters, plane mirrors, an objective lens, a focus positioner, a tracking positioner, collecting lenses and detectors. These components are typically incorporated in one optical source/detector assembly (sometimes called an optical head).
A typical optical head measures several centimeters on a side and may weigh about 100 grams. Owing to its size and weight and the nature of the focus and tracking mechanisms, it is not practical to attempt to move such optical heads at high speeds and acceleration rates. This limitation has seriously reduced the ability of optical data storage systems to perform many random accesses to data per unit time. Typical average access times reported for state of the art optical data storage products are in the range of 50-500 milliseconds. When used with an appropriate actuator motor, the present invention reduces the average access time to less than 5 milliseconds.
One, fast, random-access, linear positioner for an optical data storage device is described in part in U.S. Pat. No. 4,322,838. This positioner includes a parallelogram suspension with the objective lens mounted on the free link. Light beam focus correction located remotely from the parallelogram suspension, "upstream" of the positioner mechanism along a light beam, is also disclosed.
A rotary positioner for an optical disc memory is described in a technical article entitled "Low Inertia Scanner Performance and Selection" by D. Alan, J. Montague and B. Brosens published in Electro-Optical Systems Design, Vol. 10, No. 11, pp. 48-52, in November 1978. The rotary positioner described incorporates rotation of the actuator arm about the axis of the light beam.
An optical source/detector assembly constructed according to the principles of the present invention includes both focus and track following control apparatus remote from the moving parts of the coarse rotary positioner (also called an actuator). Since it is stationary, the size and mass of such an optical source/detector assembly do not affect the speed of the actuator or access time of the optical data storage device. The moving parts of the actuator can therefore be made very stiff and light weight (i.e., low inertia), characteristics which contribute to high speed controlled motion.
Although two-stage rotary positioners are common in the art, in this system the stages are mechanically independent. The coarse positioner for long accesses across many tracks does not move the fine positioner which is designed for short accesses and track following. As a result, the size and mass of the fine positioner are not critical because it is not accelerated by the coarse positioner mechanism. The focus positioner is also mechanically independent of the coarse positioner, resulting in similar design advantages.
Coarse positioning of the focused spot in the present invention is provided by a rotating arm attached to a torque motor. Since the arm can be small, having low inertia, very high performance torque motors of the kind employed in galvanometers may be used. The design of the optical path within the rotating arm eliminates most of the yaw or rotation of the focused light beam or beams.
The optical and mechanical components of the present invention can be arranged so that any desired region of an arbitrarily large disc can be accessed. If desired, optical heads of conventional design can be used without alteration in conjunction with such optical and mechanical components.
By making the arm of the present rotary actuator low inertia and mechanically rigid, the bandwidth of the coarse positioner is increased. Increased bandwidth allows the coarse positioner to follow the large amplitude low frequency components of radial run-out of the disc. Thus, more run-out can be tolerated and/or higher disc rotation rates can be achieved using the actuator of the present invention. Thus, in some configurations, the present invention facilitates removable media and smaller rotational latency times.